1. Field of the Invention
The present invention relates to a sheet processing apparatus for performing sorting, binding, stacking, and the like operations for sheets being discharged from an image forming apparatus or the like. More specifically, the invention relates to a sheet processing apparatus having a punch unit for punching a hole(s) into a sheet(s).
2. Description of the Related Art
Conventionally known sheet processing apparatuses includes a type that provides a post-process including, for example, sorting, binding, stacking, and hole punching to post-image-forming sheets being discharged from an image forming apparatus.
In addition, known punching method for punching a punch hole(s) into a sheet(s) in a sheet processing apparatus of the type described above include, for example, a method that stops a sheet being discharged at a constant interval from an image forming apparatus and performs punch processing of the sheet one by one.
Further, there is a known method wherein a punch unit provided and constituted of punches and dies arranged on a sheet conveying path is provided, wherein a sheet conveyance speed and a punch processing speed are synchronized to each other thereby to perform punch processing without stopping the sheet being convey, whereby the operation is performed without causing sheet processing time to be increased depending on presence or absence of the operation of the punch processing.
According to the conventional techniques, a lead edge or trail edge of the sheet being conveyed is detected, and the timing of hole punching is changed in accordance with the detecting result to thereby adjust the punching position in the sheet conveyance direction.
There is an other known method for adjusting the punching position, wherein an edge of a sheet being conveyed is detected, and a punch unit is moved to thereby move the sheet, which is being conveyed, in the sheet-width direction. Thereby, the punching position is adjusted in the direction (sheet-width direction) intersecting with the sheet conveyance direction vehicle-body width direction.
As documents disclosing related conventional examples, there are, Japanese Patent Application Laid-Open No. 03-190696, No. 06-135620, and No. 10-194557.
However, problems occur with the sheet processing apparatus wherein the sheet conveyance speed and the punch processing speed are synchronized to each other, and the punch processing is performed on a plurality of portions along a line parallel or perpendicular to the sheet conveyance direction without stopping the sheet being conveyed. The problems are that when a sheet is fed into the punch unit in the state of the paper being skewed (state of the paper being skewed with respect to the conveyance direction), the punch processing is performed on the sheet in the state of the paper being kept skewed, whereby a plurality of punch holes are punched in the sheet along a skewed line on the sheet. The tendency of such skew especially increases in a case where a conveyance path from the image forming apparatus to the punch unit is long.
Several methods have been proposed as methods of correcting such skew as described above. For example, a register-loop correcting method has been proposed. According to this method, the lead edge of a sheet is abutted against a nip of a stopping roller pair thereby to form a deflection of the sheet, and the lead edge of the sheet is then placed along the nip of the roller pair by utilizing elasticity of the sheet, thereby collecting skew.
As another method, there is a shutter skew correcting method wherein a shutter member for stopping a sheet lead edge is provided so as to be pulled back in a sheet conveyance path, and the shutter member is pulled back from the sheet conveyance path after the sheet lead edge is abutted against the shutter member, thereby to correct the skew.
In recent years, with increased speeds of image forming apparatuses, it is demanded that a sheet-to-sheet interval (inter-sheet interval) is even more reduced thereby to enhance the throughput (work volume to be processed within a fixed time).
However, in the case that the above-described correcting method is adapted for use with the conventional sheet processing apparatus that performs the punch processing, a sheet being conveyed needs to be once stopped, so that the time required for the punch processing is significantly increased. Especially, compared to the case where the punch processing is not performed, productivity is significantly diminished in the case where the inter-sheet interval is short.